Social anxiety disorder, also known as social phobia, is a common (lifetime prevalence 12%) (Kessler et al., Arch Gen Psychiatry 62:593-602, 2005), frequently disabling disorder (Stein and Kcan, Am J Psychiatry 157:1606-1613, 2000) whose pathophysiology is poorly understood (Charney, Am J Psychiatry 161:1-2, 2004). Generalized social anxiety disorder (GSAD) is a more pervasive, chronic, heritable subtype of the disorder with a lifetime prevalence of 4-5% (Schneier, BMJ 327:515-516, 2003; Stein and Gorman, J Psychiatry Neurosci 26:185-189, 2001). Serotonin reuptake inhibitors (SRIs), which refer to selective serotonin reuptake inhibitors (SSRIs) and norepinephrine-serotonin (or “dual”) reuptake inhibitors (NSRIs), are the most widely used, evidence-based treatment for GSAD (Stein et al., Cochrane Database Syst Rev CD001206, 2004) but approximately 40% of patients fail to derive an adequate therapeutic effect (Van Ameringen et al., Expert Opin Pharmacother 6:819-830, 2005). Explanations for this heterogeneity of response are lacking but could, theoretically, involve individual differences in brain serotonin metabolism. There is currently no existing method for determining the likelihood of a successful response to SRIs in individuals with GSAD. A need exists to determine the likelihood that an individual with GSAD will respond to treatment with SSRIs so that appropriate treatment protocols can be provided to subjects with GSAD. This invention is directed to this and other important ends.